The present invention concerns a method in a head box of a paper machine for controlling distribution of fiber orientation in a paper web in a transverse direction of the machine, in which a transverse speed component of a discharge jet is regulated.
Further, the present invention concerns a device intended for carrying out this method in a head box, and which comprises a turbulence generator which is provided with a perforated plate in the transverse direction of the paper machine, to which plate inlet sides of pipes in a battery of pipes are attached, and in which outlet sides of the pipes of the battery of pipes are fixed permanently or stationarily to one another, so as to open into a discharge duct of the head box.
As known in the prior art, the discharge flow of the pulp suspension in a head box must have a uniform velocity in the transverse direction of the paper machine. By the same token, it has been known that in this flow, a detrimentally high transverse velocity may occur. Particularly in the lateral areas of the web, this transverse velocity has been detrimental, for example, by forming increased waves at the edges. The distribution of velocities noted above must be within certain limits in order that a paper can be achieved which is homogeneous across an entire width of the web with respect to grammage, formation, and strength properties, and in order that only a minimum proportion at the edges of the web will have to be cut off.
Some laser printing methods which were developed in recent years, such as sheet-heating copying and continuous-form heating copying, have imposed ever higher and novel requirements on the uniformity of the structure of fine paper to be printed by means of such methods. This is mainly due to the very rapid and intensive heating of the sheet which takes place during the printing process. An especially burdensome requirement is imposed in that the main axes of the directional distribution (orientation) of the fiber mesh in the paper, should coincide with the directions of the main axis of the paper, and that the orientation should be symmetric with respect to these axes.
The above problems have been studied. Thereat, it has been learned that the symmetry required from the fiber orientation, requires that in the discharge jet, a transverse velocity of about 2 to 3 cm/s in the transverse direction of the web, is not exceeded in any part of the web. Since the transverse velocity already arises in the discharge duct along with attenuation of an uneven main flow profile, the main attenuation must be directed at uniformity of the profile of velocity in the flow direction after the turbulence generator. Even if it were possible to dimension the distribution system in the head box fully correctly, and even if this distribution system and the turbulence generators could be manufactured so accurately that the imposed requirement is met, an apparatus manufactured in this manner would become commercially unprofitable and unfeasible because of its high cost.
In the Valmet published FI Patent Applications Nos. 75,377 and 70,616 (corresponding to U.S. Pat. No. 4,687,548), a method is described for controlling the distortion of the fiber orientation, in which flows of medium are passed to both of the opposite lateral parts of the flow duct in the head box, and by means of which the distortion of the fiber orientation is controlled, these flows of medium consisting of pulp suspension flows which are passed into the lateral passages placed facing the turbulence generator, which is situated ahead of the slice part of the head box in the direction of flow in the head box, and the magnitude and/or mutual proportion of these flows being regulated so as to control the distortion of the fiber orientation by producing a transverse flow speed in the discharge flow in the head box by means of these flows, this transverse flow speed compensating for some of the distortion of the fiber orientation. These by-pass flows of pulp suspension are regulated by means of regulation valves fitted in the by-pass flow pipes.
However, a drawback in the use of these valves, that they are susceptible to being blocked, and pulp clods may be gathered in them, which, when the clods begin moving, spoil the web or may cause blocking of flow ducts or other disturbances in operation.
The use of these regulation valves also involve the drawback that, by these means thereof, it is difficult to accomplish a range of regulation that is as wide as is necessary. It is a further drawback of the use of the regulation valves that the difference in pressure between the distributor beam and the slice cone in the head box is frequently too small for reliable control. Another drawback in the regulation method based on the use of regulation valves, involves difficulties in making the method compatible with remote control or computer control.